Transmission Line Question (Yet Another)

schmeet

Member
2008-03-06 4:28 pm
Ok then,

i understand the basic principles of transmission lines and how they work. My question is, could you put two or more speakers onto the front baffle of a TL (sharing the same "pipe"). i have never seen this done anywhere so it leads me to believe that it wont work.

My initial thoughts are that it might actually help to reduce the higher resonant peaks, in teh same way as moving the driver down the line, say 1/3 or 1/5 helps to reduce them, having a driver at the beginning AND 1/3 the way AND 1/5 the way may also do the same thing.

However my knowledge of ported and sealed boxes tells me that you can't just use an enclosure that works for one speaker and put another one or two into it. You ned to double the size of the enclosure, or halve the size of the enclosure if they are isobaric.


Any thoughts?

Pete
 
...i understand the basic principles of transmission lines and how they work. My question is, could you put two or more speakers onto the front baffle of a TL (sharing the same "pipe"). i have never seen this done anywhere so it leads me to believe that it wont work...

Hi,

If you put your drivers side by side, i.e. both the drivers sees the same CSA and section length will work without flaws but placing at different CSA: s makes a difference.

Quote myself from an earlier posting:

'See the animated system freedom of two degrees coupled oscillators and try imagine what happens when (n) drivers all slightly off specification is put in a long narrow box where also the acoustic load behind each driver is different but box position symmetric, hence the action from the many different ‘stretchy cords’ that must exist in a box with many drivers sharing the same volume:'

http://www.kettering.edu/~drussell/Demos/coupled/coupled.html


http://www.kettering.edu/~drussell/Demos/multi-dof/multi-dof.html


The problem with drivers at different section positions will show up in an impedance plot and in a FR plot too.

Risk exist that the drivers are working counteractive at some frequencies making the FR less smooth than necessary.

b
 

schmeet

Member
2008-03-06 4:28 pm
Ok, So if I kept the first "leg" of the line the same, i.e. not tapered where the drivers are then i could have them one on top of the other. (As two 10" drivers side by side would call for a huge box!)

GM: What does TH stand for?

So does the CSA need to be doubled or not ??
 
TH = tapped horn: http://www.diyaudio.com/forums/showthread.php?s=&threadid=97674

If you want it to have the same response as the single driver alignment it must have 2x the net Vb for two, 3x for three, etc., same as for sealed or vented. The physics doesn't change for TLs, horns, etc..

How do you know it's not 3x a single driver alignment? Regardless, obviously you can design based on the composite specs of 'X' number of drivers and not necessarily wind up with the same response as for a single driver, but if you have a specific alignment in mind you will within a very narrow margin of error.

Physics, it's the Law! ;)

GM
 
..For example. This box below uses 3 woofers in the same transmission line and that line surely isn't 3 x the normal CSA…

Agree with that but again, IMO, it’s not a good design either, look at the following simulations: picture 1(3)-3(3).

b

1(3)
 

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CarlosT

Member
2006-10-20 6:00 pm
schmeet said:
Ok then,

i understand the basic principles of transmission lines and how they work. My question is, could you put two or more speakers onto the front baffle of a TL (sharing the same "pipe"). i have never seen this done anywhere so it leads me to believe that it wont work...

Well...it worked OK enough for Mr. D'Appolito... :D

http://www.audioxcel.com/audioXpress Thor Review.pdf

I think the biggest factor to consider is that the Vas has to be doubled in the calculations like in just about every dual-driver design. This article is fascinating and you actually get a little teaser Augsperger TL table excerpt. Mr. D'Appolito really made this project sound easy...even throwing in some looks over function fudge factor into the whole thing.
 
Yeah, but there's a slight problem with that though: the Thor cabinet is rubbish. Major alarm bells ring every time you read the details, and having run it through MathCAD, it simply confirmed what we expected, which is that it's basically a near perfectly damped TL, with an FR mirroring the IB curve. There is no extra gain at all, unless you rip all the stuffing out of the last 10in or so of the line, when it approaches the claimed measured curve (note also that the measurements in that article have been done & presented in a rather odd fashion). That's why Dave & I did a few alternative enclosures for it: to give owners the chance to actually get the promised performance. Most ended up changing the original XO, which wasn't up to much either according to a large percentage of owners, but I digress.
 
Well, what 'set off alarms' for me was while not having a clue who's/what design it was other than assuming it was the builder's when 'BrianGT'? brought a magnificently done pair to an Atlanta DIY MEET some years ago and ~30 sec. into a demo I concluded that my somewhat cruder 'redneck' version of 'rubbish' was too kind a word to describe its (lack of) ability to ~realistically reproduce music in any venue without significant acoustic gain. Indeed, other than the builder's obvious attention to detail, 'labor of love' level of woodworking and expensive looking drivers, about the only positive thing I could say before making a hasty exit was that the highs were as good as any I'd ever heard from a point source tweeter, though that's not worth much since I've never bothered to research them for my own use.

GM
 

CarlosT

Member
2006-10-20 6:00 pm
I'm sorry...you're saying that Brian did a pair of Thors and "fixed" the "errors" in the apparent "rubbish"? :D

The article made some good reading anyway and we obviously all know that D'Appollito knows speakers. Maybe art got in the way of science this time? That's saying that SEAS built these great, complex drivers and then chose the wrong mad scientist to build a "promo" cabinet? The parts themselves...nevermind the "rubbish" cabinets still go for $1,365 for the pair kit. That's a lotta hardware $$$ to put into "rubbish" cabinets or really any cabinet for that matter.
 
CarlosT said:
That's saying that SEAS built these great, complex drivers and then chose the wrong mad scientist to build a "promo" cabinet?

No question that Joe D'Appollito is a very knowledgable speaker designer. But in my opinion, he blew the Thor TL design. If you read his article carefully and know what to look for you can find where he made errors. Basically, the enclosure is too small so the bass is choked and never develops to the potential of the drivers.
 
CarlosT said:
I'm sorry.........you're saying that Brian did a pair of Thors and "fixed" the "errors" in the apparent "rubbish"?

The parts themselves...That's a lotta hardware $$$0000000.........

?? No, AFAIK Brian made a faithful reproduction and it was an expensive waste of time/$$ IMO, but then I fall outside the norm when it comes to what constitutes accurate reproduction, so as always YMMV. I didn't read the article until years later to get the measured specs, dims, so that I could design a better cab for some folks who appeared interested, but AFAIK none were built and this 'wheel' was re-invented by Dave/Scott somewhat later, designing/testing? more than one alignment IIRC.

WRT parts cost, from an inflation POV it's dirt cheap compared to what we scrimped/saved for decades ago for decent performing drivers/XO components, so relatively speaking the kit seems a bargain to me, it's just a too compromised cab design IMO. That, and like most cone/dome speakers, its limited dynamics is best suited for small apartment/condo dwellers, but then I imagine these same folks would mostly consider them too large, so not sure where his mindset was WRT trade-offs on this one.

GM
 
I suspect Thor sells primarily because of the name, and also, that article makes very plausible reading. It's only when you look at the details that things go pear-shaped, and even then, that depends on you knowing a bit about TL design, which many (most?) people simply don't have. Hifi: especially commercial or semi-commercial hifi, is frequently just smoke & mirrors anyway. Look at the (frequently very persuasive-sounding) drivel spouted by cable companies in order to flog their carpet-pythons.

I stand by my comment. The Thor cabinet is rubbish. In all honesty, I was horrified that someone with J.D.'s reputation would sign this piece of junk off. It's primarily the volume & damping that's the problem: not enough in the first case, too much in the second. But as Martin said, there are quite a few other facets too, if you know what you're looking for. In fact, one of them is related to the query here: drivers at different points in the line reducing midbass ripple. Debatable. If correctly positioned, and assuming both behave identically, it's plausible enough. But they'd have to be very carefully positioned speficially for that duty, & in Thor's case, they're not. That said, it's so over-damped, it doesn't make any difference... ;)

I didn't know you'd come up with some different boxes for the drivers Greg -can't say I'm surprised though. Wish I'd known -I could have pointed people staight to yours. :) Yeah, Dave & I did 3 variations in MathCAD, a larger tapered line, & 2 MLTLs; they're over in the Thor thread on the loudspeakers section IIRC.